I bought the engine and tranny out of an '05 Avalanche. I did a VIN search and found the engine is a "L59".

I see stuff advertised for "Vortec" or "LS1" or "LQ4" and the like, but I can't find anything for my "L59". Is there another engine designation I should look for?

 

Look here
GM Vortec engine - Wikipedia, the free encyclopedia

The Vortec 5300, or LM7/L59/LM4/L33, is a V8 truck engine. It is a longer-stroked (by 9 mm) version of the Vortec 4800 and replaced the 5700 L31. L59 denoted a flexible fuel version of the standard fuel LM7 engine. Displacement is 5.3 L (5,328 cc (325.1 cu in)) from 96.01 mm 3.78-inch (96 mm) bore and 92.00 mm (3.6 in) stroke 3.622-inch (92.0 mm) stroke. Vortec 5300s are built in St. Catharines, Ontario and Romulus, Michigan.